Kaposi's sarcoma (KS) is the most frequent cancer in HIV-infected individuals and the most common malignancy in several countries in subequatorial Africa. KS is largely incurable with current therapeutic options, and while KS is caused by the Kaposi's sarcoma herpesvirus (KSHV), no effective virus-specific therapies exist. KSHV also causes primary effusion lymphoma (PEL) and multicentric Castleman's disease (MCD), which are also lethal and incurable diseases. Our overarching goal is to develop novel targeted therapies for KSHV-associate diseases. Experimental evidence indicates that the KSHV-encoded protein vFLIP is a potential therapeutic target. vFLIP is responsible for NF-kB activity in PEL cells, and elimination of vFLIP in these cells results in tumor cell apoptosis and autophagy. Transgenic expression of vFLIP in B cells induces tumors of B cell origin in mice. These observations support the role of vFLIP as a viral oncogene, and indicate that there is addiction to this viral protein in PEL cells, supporting the notion that inhibition of vFLIP is a viable therapeutic approach for the treatment of PEL. There is also substantial evidence that vFLIP plays a role in the pathogenesis of Kaposi's sarcoma, as it is expressed in lesional cells and has a variety of transforming effects when expressed in endothelial cells. Identification of inhibitors of KSHV vFLIP will be facilitated by the development of improved screening methodologies, adequate animal models and a deeper understanding of vFLIP-mediated oncogenesis. We have made progress towards these goals by identifying small molecule inhibitors of vFLIP, and developing vFLIP conditional knock-in mice with a robust tumor phenotype. We propose to perform additional preclinical studies to develop and test vFLIP inhibitors through the following specific aims: 1) characterize small molecule inhibitors of vFLIP already identified, perform structure-activity relationship analysis, and identify improved next generation inhibitors of vFLIP; 2) evaluate other vFLIP interactions and assess their relevance to vFLIP function and viral pathogenesis; and 3) test the most promising inhibitors of vFLIP in animal models of KSHV vFLIP malignancy. Through these aims we expect to identify robust vFLIP inhibitors and acquire the preclinical information necessary to pursue subsequent development for clinical testing of the best candidate. In addition, we expect to gain further insights on vFLIP structure and function, develop improved animal models, thereby advancing our understanding of the pathobiology of KSHV-associated malignancies.